In order to understand the RG flows in the coupling space, we start by finding out fixed points of the flow and then expand the couplings about the fixed points. This leads to a classification of the couplings into relevant, marginal, and irrelevant depending on their behavior in the infrared (IR). In the IR, relevant couplings diverge away from the IR fixed points as we integrate out degrees of freedom while irrelevant couplings converge towards the IR fixed point. The subspace of relevant couplings is usually finite-dimensional, and hence theories in the IR are characterized by only a finite number of relevant couplings. Such IR behaviour defines the notion of universality classes [1]}, [2]}, [3]}, [4]}. The set of relevant couplings can be used to determine some of the critical exponents, and scaling relations between them further allows us to determine all the critical exponents [5]}, [6]}, [7]}, [3]}, [4]}.
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